The ability to turn an electrical switch on or off automatically in one's absence or in response to certain changes in the local environment presents certain advantages. For example, using a timer control to turn lights, appliances and the like on and/or off at specific times can enhance the security of a home or office. Energy can be conserved by such a program, and associated costs reduced accordingly. Ease and convenience are additional advantages.
In general, automatic switch control involves devices coupled to the existing AC circuit. These include timer control and time delay control devices, as well as devices incorporating certain sensors that respond to changes in the local environment. Such sensors include passive infrared (PIR) sensors, which respond to changes in heat (including body heat), and cadmium sulfide (CdS) photosensors, which respond to changes in light. In addition, remote control using, for instance, infrared or radio signals can be used for automatic switch control.
In general, the many existing electronic and mechanical products for automatic switch control fall into two broad categories: permanently installed devices and plug-in/add-on devices.
In the former case, the existing switch is removed from the site where automatic switch control is desired, and an automatic control device with a switch is installed in its place. In addition to being a waste of the existing switch, there are other drawbacks to this approach. Re-wiring is necessary to install such a device, or to remove it, if so desired at a later time. With re-wiring, there is always a risk of electrical shock. This risk, plus the knowledge of electrical wiring and the work required for installation, can often discourage an ordinary person from using a permanently installed automatic switch control device. Moreover, the power consumption of lighting or appliances controlled by such a device may be limited by the device itself. Usually, the usable range is limited to 100 W to 600 W.
In contrast, the plug-in/add-on devices of the second broad category of automatic switch control devices do not require rewiring. Here, the light or appliance to be controlled automatically is plugged into a separate control module which is in turn plugged into a wall outlet. The control module may incorporate a timer, or may sense changes in the environment. These may be changes in heat, light or sound (for instance, hands clapping). Plug-in/add-on devices are limited to plug-in applications. In addition, they can sometimes be cumbersome. In use, one socket of a standard wall outlet must be reserved for the control module, and the module sometimes blocks the second socket.